Loosely related... what's the BSFC of our engines roughly, or comparable small automotive diesels?
Now that's the right question to ask... minimize BSFC (brake-specific fuel consumption) and you will see mileage being maximized.
I wouldn't be quick to accept the notion that the most efficient engine would no naturally aspirated. I feel in my gut that the turbo would be more efficient. You see when looking at the BSFC maps of the naturally aspirated diesel and 1.6lTD, that although they both have a similar minimum BSFC, the 1.6lTD has a broader "plateau" of minimum BSFC. So what I think is, when you're actually driving it (the TD) you will see better real-world mileage. From a thermodynamics standpoint it makes sense that the turbo could add efficiency, because it can make more torque (IE: smoke-free) so can be run at lower RPMs thereby reducing engine RPM-related friction (IE: bearing drag, etc). The only cost is a little extra backpressure, but I think that's more than made up for by the engine thinking it's bigger as a result of the boost pressure. The turbo recovers some portion of energy that would normally be lost as heat out the exhaust. Granted, the turbo is not perfectly efficient itself, but I think it's true that a turbocharged engine is more efficient than a naturally aspirated engine with enlarged displacement (so the two engine being compared develop same amount of power or torque output.) This is particularly true with a turbo-diesel, as CR doesn't need to be reduced.
There are BSFC plots (of RPM vs load) in some of the SAE papers... off-hand I only know that the TDI engine broke below the 200 g/kWh level.
Edit: I looked up the BSFC maps - The 1.6l diesels are around 260 g/kWh.
For 5-cyl IDI TD motor, see figure 54: (note:bsfc contours are hard to read, but range from 261 to 562)
For 4-cyl 1.6lTD motor, see figure 36: (note:bsfc contours range from 264 to 977)
And those of you with the 1.5lD SAE paper, can see a BSFC plot for this motor on page 111 (figure 55.) The minimum contour is 250 g/kWh, at about 80% throttle, between 2250-2500RPM.